linux-kernel - Re: [PATCH v5 12/16] PCI: Add pci_iomap_host_shared(), pci_iomap_host_shared

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Date:   Sat, 9 Oct 2021 13:39:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        James E J Bottomley <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S . Miller" <davem@...emloft.net>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 12/16] PCI: Add pci_iomap_host_shared(), pci_iomap_host_shared_range()

On Sat, Oct 9, 2021 at 2:53 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Oct 08, 2021 at 05:37:07PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> >
> > For Confidential VM guests like TDX, the host is untrusted and hence
> > the devices emulated by the host or any data coming from the host
> > cannot be trusted. So the drivers that interact with the outside world
> > have to be hardened by sharing memory with host on need basis
> > with proper hardening fixes.
> >
> > For the PCI driver case, to share the memory with the host add
> > pci_iomap_host_shared() and pci_iomap_host_shared_range() APIs.
> >
> > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>
> So I proposed to make all pci mappings shared, eliminating the need
> to patch drivers.
>
> To which Andi replied
>         One problem with removing the ioremap opt-in is that
>         it's still possible for drivers to get at devices without going through probe.
>
> To which Greg replied:
> https://lore.kernel.org/all/YVXBNJ431YIWwZdQ@kroah.com/
>         If there are in-kernel PCI drivers that do not do this, they need to be
>         fixed today.
>
> Can you guys resolve the differences here?

I agree with you and Greg here. If a driver is accessing hardware
resources outside of the bind lifetime of one of the devices it
supports, and in a way that neither modrobe-policy nor
device-authorization -policy infrastructure can block, that sounds
like a bug report. Fix those drivers instead of sprinkling
ioremap_shared in select places and with unclear rules about when a
driver is allowed to do "shared" mappings. Let the new
device-authorization mechanism (with policy in userspace) be the
central place where all of these driver "trust" issues are managed.

> And once they are resolved, mention this in the commit log so
> I don't get to re-read the series just to find out nothing
> changed in this respect?
>
> I frankly do not believe we are anywhere near being able to harden
> an arbitrary kernel config against attack.
> How about creating a defconfig that makes sense for TDX then?
> Anyone deviating from that better know what they are doing,
> this API tweaking is just putting policy into the kernel  ...

Right, userspace authorization policy and select driver fixups seems
to be the answer to the raised concerns.